1. Field of the Invention
The present invention relates to a structure of a radial turbine scroll which is used with an exhaust turbosupercharger of a relatively medium- to small-sized internal combustion engine and which is constructed such that an operating gas from an engine (internal combustion engine) is led to flow in a radial direction from a spiral scroll formed in a turbine casing into turbine moving blades of a turbine rotor, which is positioned on the inner side of the scroll, to act on the turbine moving blades, and then led to flow out in an axial direction, thereby rotatively driving the turbine rotor.
2. Description of the Related Art
FIG. 6 is a sectional view taken along the line of axial center, illustrating a structure of an engine exhaust turbosupercharger.
In FIG. 6, reference numeral 1 denotes a turbine casing. A spiral scroll 4 is formed in the turbine casing 1, and a gas outlet passage 5 is formed at the inner periphery of the turbine casing 1.
A bearing housing 9 is fixed to the turbine casing 1, and a compressor housing 6 is fixed to the bearing housing 9.
A turbine rotor is denoted by reference numeral 10. A plurality of turbine moving blades 3 is secured to an outer periphery of the turbine rotor 10 at regular intervals in the circumferential direction.
The compressor housing 6 accommodates a compressor 7, and a diffuser 8 is provided at an air outlet of the compressor 7. A rotor shaft 12 connecting the turbine rotor 10 and the compressor 7 is supported by the bearing housing 9 through the intermediary of two bearings 11 and 11. The center of rotation is denoted by 20Z.
FIGS. 7(A), (B), and (C) are sectional diagrams of the scroll 4 of the turbine casing 1 and a W-W sectional diagram (FIG. 7(C)) thereof.
In the exhaust turbosupercharger, an exhaust gas from an engine enters the scroll 4, circumferentially moves along the convolution of the scroll 4 to flow into the turbine moving blades 3 from an end surface of an inlet 4c on the outer peripheral side of the turbine moving blades 3, further flows in the radial direction toward the center of the turbine rotor 10 to carry out expansion work on the turbine rotor 10, and then flows out in the axial direction to be discharged outside through the gas outlet passage 5.
At the time of the aforesaid operation, as illustrated in FIGS. 7(A), (B), and (C), the scroll 4 is formed in a spiral shape in the turbine casing 1, and a tongue portion 21 is formed on the inner periphery of a gas inlet portion of the scroll 4. The tongue portion 21 needs to have a thickness of approximately at least 3 mm, because the turbine casing 1 is a casting.
Hence, a wake (low-speed area) 30 at the tongue portion occurs when the exhaust gas flows. The wake 30 is larger in FIG. 7(B) wherein the tongue portion 21 is thicker than in that in the case of FIG. 7(A), so that the deterioration of the performance of the turbine caused by the wake 30 at the tongue portion 21 is worse accordingly.
The one disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-120303) has a tongue portion formed on the inner periphery of a gas inlet portion of a scroll. The sectional area of a flow passage adjacent to a flow immediately below the tongue portion is set to be smaller than the sectional area of a flow passage at a tongue portion end by the dimension equivalent to the thickness of the tongue portion in the width direction, thus permitting a reduction in the wake occurring at the tongue portion.
As described above, in the conventional exhaust turbosupercharger, as illustrated in FIGS. 7(A), (B) and (C), the wake (low-speed area) 30 at the tongue portion occurs at the time of the flow of an exhaust gas, and the wake 30 increases as the tongue portion 21 is thicker. The occurrence of the wake 30 at the tongue portion 21 leads to the deterioration of the turbine performance.
More specifically, the wake (low-speed area) 30 is attributable to the flow of a gas moving from a radially outer side toward a radially inner side, and the flow of the exhaust gas heading toward the inner side is smaller in the case where the tongue portion 21 is thinner, as illustrated in FIG. 7(A), resulting in less deterioration of the turbine performance. In this case, however, the thermal stress increases since the tongue portion 21 is thinner.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-120303